This invention relates to one-piece plastics closures for containers, that is to say, to container closures which have been moulded from polymeric material so as to incorporate a formation adapted for sealing engagement with a container to which the closure is fitted. This is to be contrasted with closures having a sealing wad, gasket or other feature which as a post-operation is fitted or formed in the closure after the latter has been moulded; closures of the latter kind are often referred to as "two-piece", even though the two components, i.e. the plastics moulding and the sealing feature, may be intimately joined together.
As is manifest from the many proposals for one-piece plastics closures which exist in the patent literature, it has long been realised that one-piece closures can provide considerable cost benefits over their two-piece counterparts. Only one moulding operation and the associated equipment is needed; moreover, there is no requirement to assemble components together or to position them in relation to one another for assembly.
Carbonated beverages are generally considered to represent one of the most onerous possible applications of plastics closures. The retention of high container pressures over long periods of time is difficult to achieve, and the difficulty is made greater by variations in the conditions to which the container may be subjected during transport and in storage; top-loading pressures and temperatures are two parameters to which the sealing efficiency of one-piece plastics closures may be particularly sensitive.
GB Patent specification No. 1539022 was proposed by the inventor of the present application for a container of a pressurised product. It has a flexible sealing formation which projects generally radially inwardly of the closure towards its free edge, and in the fitted position of the closure it is mechanically urged firmly and downwardly against the rim of the container neck by abutment of its upper surface by the generally annular bottom surface of a projecting support ring which is moulded to project from the underside of the generally plane closure panel or crown of the closure body.
In Patent specification No. 1539022, the pressure of the product in the container is allowed to act upon the sealing formation so as by pneumatically forcing it against the container rim to enhance the seal achieved. For that purpose the support ring is segmented, the spaces between the segments allowing the gas in the container headspace access to the top of the sealing formation. Outside the support ring (but above the container rim), the closure body is relieved from engagement with the sealing formation, so forming an annular chamber to which the product pressure is communicated. The support ring and the relieved surface of the closure body accordingly together form what may be regarded as a mechanical abutment and pneumatic reaction surface, for cooperation with the sealing formation. The support ring provides the abutment function of this surface; the relieved surface provides the reaction function of the surface, and is operative radially outside the support ring.
As previously indicated, one of the parameters of a carbonated beverage container to which many of the one-piece plastics closures proposed hitherto have been particularly sensitive is the top loading which may be applied to the container during transit and display, for example by other such containers placed on top of it. Because its seal interface with the container neck is perpendicular to the applied forces, the closure described in Patent specification 1539022 has been found in practice to be sensitive to top loading, and Applicants believe that it is incapable of meeting the present top-loading requirements of major carbonated beverage manufacturers.
In the closure illustrated in Patent Specification No. 1539022, the sealing formation is shown to extend for a considerable distance inwardly across the container rim, and moreover its attachment to the container body is located above the container rim radially within the outer periphery of the latter. Because of these spatial relationships little or no radially directed movement of the sealing ring across the container rim is required to occur as the closure is being fitted, and there will be correspondingly little danger that the sealing formation will be buckled or otherwise irregularly distorted by the support ring. Therefore, whilst sealing efficiency is assured in the absence of top loading, the closure of patent specification No. 1539022 is highly reliant upon sealing generally transversely to the axis of the closure; it accordingly has a substantial sensitivity to top loading as previously mentioned.
In Patent specification No. 1539022 the closure is arranged to be an interference fit (at its surface 41) with the outer surface of the container rim, but sealing at this locality cannot be relied upon because of the substantial variations which may exist in the relative dimensions of the container and the closure, especially if the container is made of glass and under elevated temperature conditions.